Low loss, single mode, optical Y-junctions consisting of planar waveguides in semiconductor or dielectric material are important components for optical communications and optical signal processing systems. Conventional devices using single-mode Y-junctions include switches (W.E. Martin, "A new waveguide switch/modulator for integrated optics," Appl. Phys. Lett., 26 pp. 562-564 (1975)), modulators (Martin, supra), samplers (M. Izutsu, H. Haga, and T. Sueta, "Picosecond signal sampling and multiplication by using integrated tandem light modulators," J. Lightwave Technol. LT-1 pp 285-289 (1983)), multipliers (Izutsu, supra), A/D convertors (R.A. Becker and F.J. Leonberger, "2-bit 1 Gsample/s electrooptic guided wave analog-to-digital converter," IEEE J. Quantum Electron. QE-18, pp 1411-1413 (1982)), and logic gates (A. Lattes et al.,"An ultrafast all-optical gate," IEEE J. Quantum Electron. QE-19, pp 1718-1723 (1983)). A significant problem associated with these devices concerns the loss of optical power in the guided mode at the Y-junction. Another difficulty relates to the coupling of optical fiber to the input and output ends of the planar waveguide.
A typical Y-junction as shown in FIG. 1 is formed by matching a single straight waveguide 10 with two other waveguides 11 and 12 that are each positioned at a half-angle .alpha. from the longitudinal axis of the first waveguide. The widths of the waveguides and the difference in index of refraction between the guiding and cladding layers are controlled so that the waveguides will support only one mode at a desired optical wavelength .lambda.. Although the bend between the first waveguide and each of the angled waveguides is often discrete, other Y-junctions may have a smoothly varying radius of curvature, as disclosed by M.W. Austin in "GaAs/GaAlAs curved rib waveguides," IEEE J. Quantum Electron. QE-18, pp 795-8OO (1982) or even a more complicated discrete bend. In these conventional Y-junctions, the percentage of light lost at a junction in the waveguide depends upon the difference in the indices of refraction and .alpha.. For a single mode waveguide the loss always increases with increasing .alpha..
When used in optical communication and optical signal processing systems, planar waveguide devices are almost always fiber-pigtailed to conveniently couple light into and out of the waveguide structure. In particular, FIG. 1 shows a fiber input line 13 coupled to waveguide 10, and fiber output lines 14 and 15 coupled to waveguides 11 and 12, respectively. While coupling losses as low as 2 dB/facet have been reported by M. J. O'Mahony in "Semiconductor Laser Optical Amplifiers for Use in Future Fiber Systems," J. Lightwave Technol. LT-4 pp 531-544 (1988), typical coupling losses are about 5 dB/facet. Fiber coupling to the waveguide requires that the length L of the waveguide, measured from the beginning of the angled section, must satisfy the equation L.gtoreq.d/(2 tan.alpha.), where d is the outer diameter of the fiber. The outer diameter of most telecommunications grade fibers is 125 .mu.m. Another consideration is that losses due to material absorption and scattering from waveguide imperfections increase as the length increases. Since it is difficult to grow the device in view of these limitations, there is a trade-off between making a device longer to reduce the bend angle and thereby reduce the loss due to bending, and making the device shorter to reduce the intrinsic loss of the waveguide. Disadvantageously, longer waveguides suffer from increased absorption losses while shorter waveguides exhibit higher junction losses due to the larger bend angles required.
In addition to simple radiation loss at a bend in a single mode waveguide, planar waveguides with branches suffer from a number of other problems. For example, the light lost due to bending depends upon polarization state, and thus the overall device may introduce polarization dependent noise. In active (pumped) semiconductors, the index of refraction is a function of carrier density and thus the throughput of the device may depend upon signal strength (saturation), wavelength, or gain.
For particular waveguide parameters, such as those disclosed by L. M. Johnson, Z. L. Liau, and S. H. Groves, in "Lowloss GainAsP buried-heterostructure optical waveguide branches and bends," Appl. Phys. Lett. 44 pp 278-280 (1984), the angle at which half of the light is lost in a discrete bend (the 3 dB bend angle) in InGaAsP was found to be approximately 2.5.degree.. The length of a device with a single bend of angle 2.5.degree. would need to be at least 1.3 mm long, which is unrealistically long for conventional epitaxial growth techniques. Changing the geometry of the waveguide to a much more sophisticated bend similar to the one presented by P. D. Swanson et al. in "Low-loss semiconductor waveguide bends," Optics Letters 13 pp 245-247 (1988) can significantly increase the 3 dB bend angle to approximately 7.degree., which leads to a more realistic device of length .gtoreq. 0.5 mm. The expected bend and material loss in other semiconductor material systems of interest (such as GaAs/GaAlAs) are expected to be similar. In dielectric materials such as LiNbO.sub.3, the modes are much more loosely bound, and so half angles of less than 1.degree. are typical. The devices fabricated from these materials are grown as bulk crystals and are very nearly transparent so that long devices on the order of a few millimeters are acceptable. Typical junction losses in practical LiNbO.sub.3 devices are also approximately 3 dB.
In comparison to the energy loss experienced by a conventional Y-junction fabricated in semiconductor or dielectric materials, optical fiber couplers are able to couple light from a single fiber into two output fibers with almost no extraneous loss. For example, the extraneous loss of a good, commercially available, 1.times.2 optical coupler can be as low as 0.2 dB. However, it is extremely difficult to modulate the index of refraction through an electro-optic effect or to provide gain in an optical fiber. Furthermore, there is no known way to modulate either one of these parameters (refractive index or gain) at frequencies of efficiencies that approach those achievable by semiconductor or dielectric waveguides.